1. Field of the Invention
The present invention relates to a sweeping technology for a streak tube that can detect optical events which occur in very short time intervals.
2. Description of the Prior Art
In streak tubes, light to be measured is introduced onto a photocathode which generates a number of photoelectrons corresponding to the amount of light. The photoelectrons are accelerated and focused in an electron beam. A sweeping signal is applied to deflection plates provided in the path of the electron beam. The electron beam is deflected on the deflection plates, forming a streak image on a phosphor screen. The streak image is used to measure the strength of the introduced light.
When used in combination with a titanium sapphire laser light source for generating a light pulse at a high repeating frequency stabilized at about 100 MHz, the streak tube not only can measure extremely faint fluorescent light and the like, but can accurately accumulate faint streak images at the same position on the phosphor screen by applying to the deflection plate a sweeping signal in the form of a sinusoidal wave, synchronized with the high repeating frequency of the laser light source. Therefore, optical event can be measured in a short time duration with few jitters and a high signal-to-noise ratio.
Methods for applying a sweeping signal to the deflection plates in a streak tube that are well-known in the art include a single sweeping method, synchronization scan sweeping method, duplex sweeping method, and elliptical sweeping method. The diagrams in FIGS. 1(a) through 1(c) illustrate the single sweeping method known in the art. FIG. 1(a) shows the waveform of a synchronization scan sweeping signal; FIG. 1(b) shows the waveform of a horizontal (H) blanking signal; and FIG. 1(c) shows the movement of the electron beam over the phosphor screen. The synchronization scan sweeping signal and horizontal blanking signal are generated in synchronicity with the light pulse output from the laser light source.
The synchronization scan sweeping signal is applied to vertical deflection plates in the streak tube. Simultaneously, the horizontal blanking signal is applied to the horizontal deflection plates in the streak tube. When these signals are applied, the electron beam issued when light to be measured is introduced onto the photocathode is deflected by the electric fields formed by the vertical and horizontal deflection plates. The electron beam is scanned over the phosphor screen as shown in FIG. 1(c). When the horizontal blanking signal is at a low level and the synchronization scan sweeping signal is changing from the low level to the high level, the electron beam is swept to move across the output effective area on the phosphor screen. A streak image is therefore obtained for each period of the synchronization scan sweeping signal, in other words for each optical pulse output from the laser light source. The other conventional sweeping methods also form a streak image of the light to be measured on the phosphor screen for each pulse output from the laser light source.
However, all of the conventional streak tube sweeping methods require the use of a laser light source capable of outputting an optical pulse at a stabilized repeating frequency, because the streak images are all formed precisely at the same position on the phosphor screen. Hence, it is necessary to use a laser light source outputting an optical pulse with a repeating frequency as high as 100 MHz. However, since a streak image of the light to be measured is formed for each optical pulse output from the laser light source, streak images for different times are formed at the same position on the phosphor screen when the time required to generate the light to be measured is longer than the period of the optical pulse.
For example, when exciting fluorescent matter by an optical pulse output from the laser light source and measuring the fluorescent light emitted from the fluorescent matter, if the life of the fluorescent light is longer than the period of the optical pulse output from the laser light source, the fluorescent matter will be excited by the next optical pulse, creating new fluorescent light, before generation of the first fluorescent light has sufficiently completed. In this example, the fluorescent light cannot be accurately measured.